The present invention relates to an electrophotographic photoreceptor and more particularly to a double-layered electrophotographic photoreceptor excellent in storage stability and repeated use characteristics.
Hitherto, as electrophotographic photoreceptors, there have been known those which have a photosensitive layer mainly composed of inorganic photoconductors such as selenium, zinc oxide and cadmium sulfide.
However, these are not necessarily satisfactory in sensitivity, heat stability, moisture resistance and endurance and especially, selenium and cadmium sulfide photoreceptors are limited in production and in handling because of their toxicity.
On the other hand, electrophotographic photoreceptor having a photosensitive layer mainly composed of organic photoconductive compounds have many merits that they are relatively easy in production, inexpensive, easy in handling, and generally superior to selenium photoreceptors in heat stability and recently attract attentions in this field.
As such organic photoconductive compounds, poly-N-vinylcarbazole is well known and electrophotographic photoreceptor having a photosensitive layer mainly composed of a charge transfer complex formed from the above poly-N-vinylcarbazole and a Lewis acid such as 2,4,7-trinitro -9-fluorenone is disclosed in Japanese Patent Kokoku No. 50-10496. However, this photoreceptor is not necessarily satisfactory in sensitivity, film-formability and endurance.
On the other hand, organic photoconductors of low molecular weight represented by hydrazones and pyrazolines have been proposed. Film-formability can be considerably improved by combining these organic photoconductors with suitable binders, but improvement of sensitivity and endurance is not sufficient.
Under the circumstances, recently there have been proposed double-layered type photoreceptors where carrier generating function and carrier transporting function are born on separate substances. Employing this structure has resulted in remarkable improvements in charging characteristics and sensitivity and has given photoreceptors having sensitivity close to that of inorganic photoreceptors such as Se by combination of a carrier generation layer comprising azo pigment having a high carrier generating ability with a carrier transport layer containing a carrier transport material of hydrazone type which has a high carrier transporting ability. As a result, now the photoreceptors mainly composed of organic photoconductive compounds of these types have began to be used in the fields of copying machines and printers.
Electrophotographic photoreceptors are subjected to repetition of process comprising charging, exposing and removing of charge in copying machine and variation of initial potential after charging and residual potential after removal of charge affects the image and so must be as small as possible.
However, electrophotographic photoreceptors made of organic materials suffer from the problems that a slight amount of impurity incorporates into the materials during preparation thereof, which causes increase of residual potential and that because of inferior stability of materials per se, air oxidation or photo-decomposition occurs and results in impurity which causes increase of residual potential or decrease of initial potential. These are problems in use which are difficult to control.